1. Field of the Invention
The invention relates to a filter set for observing fluorescent radiation (also called fluorescence radiation; radiation caused by fluorescence) in biological tissue and thus relates to observing emission of light of an endogenous (produced naturally in the body) or exogenous fluorophore due to an excitation using radiation. Herein, fluorophore in general refers to a substance for which fluorescence (emission of fluorescent radiation) arises due to an excitation. The observation may optionally be performed in a living organism and thus in vivo or in an artificial environment (outside of living organisms, for example in vitro). Further, the invention relates to a medical optical system comprising this filter set and a method of selecting filters of a filter set.
2. Brief Description of the Related Art
Such filter sets in particular are employed in optical systems of medical technology, such as for example microscopes or endoscopes that are adapted for observing fluorescent radiation (e.g. fluorescence microscopes).
To release fluorescence in biological tissue a fluorescent dye is applied to a patient, for example. This fluorescent dye may be chosen such that it is enriched in tumour tissue in an enhanced concentration. For diagnosis, the tissue to be examined is illuminated with excitation radiation after application of the fluorescent dye. This excitation radiation has to be chosen in dependence of an excitation band (a spectral band of the excitation radiation) of the used fluorescent dye in an appropriate way. Due to the excitation radiation, spontaneous emission of fluorescent radiation arises in the fluorescent dye. The intensity of the fluorescent radiation depends on the used fluorescent dye, the excitation band, the intensity of the excitation radiation and the enrichment of the fluorescent dye within the tissue. The fluorescent band (spectral band of the fluorescent radiation, also called fluorescence band) of the fluorescent radiation also depends on the used fluorescent dye. The excitation bands of a fluorescent dye always lie at smaller wavelengths than the associated fluorescent bands.
In this way a tumour, for example, may be marked and localized using the fluorescent radiation.
Since the intensity of the fluorescent radiation is usually more than one order of magnitude smaller than the intensity of the excitation radiation, there is the risk that the fluorescent radiation is outshone by the excitation radiation. Consequently, excitation radiation is usually used comprising a wavelength range which does not overlap with the wavelength range of the fluorescent radiation. By filtering the excitation radiation from an observation beam path the fluorescent radiation may be separated from the excitation radiation and may be observed.
Thus, a compromise between an optimal excitation of the fluorescence (by excitation of the fluorescent dye/fluorophore) by exploiting the excitation band to a wide extent and a prevention of outshining the fluorescent radiation caused by the fluorescence (and thus of a good optical opportunity to distinguish excitation band and fluorescent band) is aimed for.
Known fluorescent dyes employable in medical technology are for example Indocyanine green, Protoporphyrin IX and Hypericin. The excitation band of Indocyanine green lies at 400 nm to 780 nm and the fluorescent band lies at about 830 nm. The excitation band of Protoporphyrin IX lies at about 400 nm and the fluorescent band lies between about 630 nm and 730 nm. Hypericin has three excitation bands at 467 nm, 550 nm, and 594 nm as well as two fluorescent bands at 600 nm and 650 nm. The preceding fluorescent dyes further exhibit, beside a high intensity of the fluorescent radiation and a sufficient distance between the respective excitation band and the fluorescent band, a good compatibility and degradability of the fluorescent dye in the human organism.
The excitation bands and the fluorescent bands of Hypericin are exemplarily shown in FIG. 1. In FIG. 1 the solid line denotes the excitation spectrum and the broken line denotes the fluorescence spectrum (also called fluorescent spectrum) of Hypericin. FIG. 1 was obtained in a cell culture medium at a concentration of 1 μM.
As an alternative to applying a fluorescent dye, a so called auto-fluorescence of the tissue may be exited caused by organism endogenous (organic endogenous) fluorescent material.
During observing fluorescent radiation in biological tissue it is desirable that, in addition to observing the fluorescent radiation, it is also possible to observe tissue which is adjacent to tissue emitting the fluorescent radiation. This facilitates, on one hand, the differentiation of diseased and healthy tissue and, on the other hand, the localisation of the diseased tissue in the surrounding healthy tissue. Otherwise there is for example the risk that, although tumour tissue may be observed using the fluorescent radiation, it may not be sufficiently localized in the surrounding tissue and that it may not be sufficiently differentiated from healthy tissue. The observation of the adjacent tissue may be performed in a colour (and thus in a spectral band), which differs from the colour of the fluorescence (the observed fluorescent band).
For observing fluorescent radiation in biological tissue a microscopy system having an illumination system and an observation system adapted to the fluorescence of Indocyanine green is known from German published application DE 103 39 784 A1, the content of which is herewith incorporated by reference.
From European Patent EP 0 861 044 B1 an apparatus for diagnosis using a reaction in biological tissue caused by a light induced photosensibilisator (photosensitizer) or by endogenous fluorescence is known. The preceding apparatus is in particular suitable for the use of Delta-Aminolevulinic-Acid (ALA) as fluorescent dye.
For the known solutions, deficiencies may occur depending on the fluorescent dye used when observing at the same time the fluorescent radiation and tissue which is adjacent to the tissue emitting the fluorescent radiation. In particular, the known solutions are suitable to only a limited extent for fluorescent dyes for which the excitation band and the fluorescent band lie very close to each other or partially overlap (as is for example the case for Hypericin).